This invention relates to series-connected combinations of two-terminal semiconductor devices, such as microwave diodes, and to methods of fabricating such combinations on a common substrate.
It is well known that the amount of power that can be obtained from a microwave diode whether operating in the avalanche effect or transferred electron effect mode, is determined by the active area of the device--the greater this area, the more power obtainable. However, as the active area of the device is increased to obtain increased power, the capacitance of the device also increases, and this in turn causes a reduction in the impedance of the device, which varies inversely with the capacitance. If the impedance of the device becomes too small, difficulties arise in matching it with the impedance of associated circuitry, and to overcome this difficulty, it is known to connect a number of individual devices together in series to obtain the required power while maintaining an acceptable value of impedance for the combination.
Hitherto this has usually been done by connecting together a number of individually packaged devices resulting in a relatively bulky combination, and requiring individual interconnections between the devices to be carried out manually, e.g. by soldering or welding.
It is an object of the present invention to provide means whereby the above-mentioned disadvantages may be overcome or at least substantially reduced.
According to a first aspect of the present invention a method of fabricating a series-connected combination of two-terminal semiconductor devices on a common substrate comprises: PA1 forming a layer of high quality semi-conductor material on the surface of a temporary substrate to provide active areas for the devices; PA1 forming first contact pattern conductors on the free surface of the high quality semiconductor layer to provide a separate first contact to this layer for each of the devices; PA1 bonding an insulating support substrate to the first contact pattern; PA1 removing the temporary substrate; PA1 forming second contact pattern conductors on the other surface of the high quality layer to provide a separate second contact to this layer for each of the devices; PA1 removing regions of the high quality layer separating the conductors of a pattern at any stage after beginning formation of the first contact pattern in order to define the device active areas so that parts of the first contact pattern are exposed when both the temporary substrate and the regions of the high quality layer have been removed, PA1 and providing interconnections between the exposed parts of the first contact pattern and parts of the second contact pattern, whereby to connect the devices together in series. PA1 fabricating a first precursor by the following procedure: PA1 forming a layer of high quality semiconductor material on the surface of a temporary substrate, PA1 forming first contact pattern conductors on the free surface of the high quality semiconductor layer to provide a separate first contact to this layer for the devices thereon, PA1 bonding an insulating support substrate overlayed by a conducting interconnection pattern to the first contact pattern so as to provide contact between the first and the interconnection patterns, PA1 removing the temporary substrate, PA1 forming second contact pattern conductors on the other surface of the high quality layer to provide a separate second contact to this layer for the devices thereon, PA1 removing part of the high quality layer to form isolated regions in the high quality semiconductor material sandwiched between the first and second contact patterns, PA1 fabricating a second precursor by the same procedure as that used for the first precursor, PA1 bonding the first and second precursors together so as to provide contact between the second contact pattern of the one and the interconnection contact pattern of the other.
Preferably the support substrate according to the first aspect of this invention is formed on its surface with a metallic pattern to which the first contact pattern is bonded before removal of the temporary substrate.
In the method according to the first aspect of the invention, the first contact pattern is preferably formed in two stages, the first stage providing discrete contacts for each of the devices defining the active area thereof, and the individual active regions of the device then being isolated from one another by exposing the active layer to an ion beam using these contacts as a mask to create areas of semi-insulating material in the active layer surrounding each of the device active regions underlying the contacts. The second stage of the formation of the first contact pattern then comprises formation of a contact pad portion for each of the device first contacts which are to be interconnected with a device second contact, each of these contact pad portions overlying at least part of the associated first contact, and an adjacent area of the ion beam isolated region of the active layer. Either following removal of the temporary substrate, or before carrying out the second stage in the formation of the first contact pattern, preferably the latter, part of the ion beam isolated region of the active layer over which the contact pad portions are formed, is removed, to enable the interconnections between the contact patterns on opposite sides of the active layer to be made following removal of the temporary substrate, the removed parts of the active layer being isolated from the device active regions.
Alternatively, the isolation of the device active regions may be performed after removal of the temporary substrate, these regions being defined by the second contact pattern which is thus used as a mask for the isolation process. The interconnections between the contact pad portions associated with the device first contacts and the contacts of the second contact pattern are formed by an additional metallization pattern.
According to a second aspect of the present invention a method of fabricating a series-connected combination of two-terminal semiconductor devices on a common substrate comprises:
The isolated regions of high quality semiconductor material are preferably formed according to the second aspect of the invention by removing the surrounding semiconductor material by etching. The etching may be performed immediately prior to or after formation of the first contact pattern by creating mesas in the high quality layer or alternatively immediately prior to formation of the second contact pattern instead of after the second contact pattern formation. However, if etching is performed with a contact pattern present on the free surface of the high quality layer, this pattern may advantageously serve as an etch mask.
In order to achieve good uniformity of device characteristics, the high quality semiconductor material of both precursors according to the second aspect of the invention are preferably formed in a single step which produces a single layer, this layer then being scribed and broken into parts for each precursor.
According to a preferred method in accordance with both aspects of the invention, the high quality layer is formed of active semiconductor material having suitable semiconductor properties capable of supporting device operation and the support substrate is formed of an electrically insulating material having a high thermal conductivity.
Preferably at least the region of the temporary substrate immediately adjacent to the high quality layer according to both aspects of the invention is removed by selective etching using an etchant to which the high quality layer is resistant. Conveniently, the substrate may comprise a first relatively thick layer of semiconductor material formed on its surface with a relatively thin buffer layer of semiconductor material on which the high quality active layer is formed, e.g. by epitaxial growth, the relatively thick substrate layer then preferably being removed by a selective etching procedure to which the buffer layer is resistant, followed by removal of the buffer layer using a second selective etching procedure to which the high quality layer is resistant.
Such a two-layer temporary substrate may comprise a relatively thick layer of GaAs and a thin epitaxial buffer layer of GaAlAs with a high quality semiconductor layer of GaAs. Alternatively, the high quality layer may be of another semiconductor material, such as silicon, epitaxially formed on a temporary substrate having a suitable selectively etchable single or multilayer structure.
The invention enables a number of individual two-terminal devices, such as microwave diodes, to be connected together in series at chip-level, i.e. in a single integrated circuit, resulting in a considerable reduction in size and in the spacing between adjacent devices providing advantages at high frequencies. The invention also avoids the need for individual packaging of the devices thereby avoiding parasitics associated with such packaging, and the reliability of the interconnections between the devices can be improved as the devices do not need to be individually bonded as in conventional series-connected arrangements. Further, because all the devices of the series combination can be formed from the same active layer, and more importantly from closely adjacent areas of the same active layer, greater diode uniformity is achieved, and increased yields can be obtained.